It is not clear if the image has been altered but Mr Daly wrote: “This is sent to me with the absolute assurance that it's a genuine picture taken by a flight attendant at American Airlines. The F/A took it to show her manager what was happening on the aircraft (757???) and why she was unhappy about it. Seems the guy paid for only one seat and the gate staff let him board.”

Comments below the blog entry illustrate the debate over how to deal sensitively with obese passengers while ensuring others have enough room.

In a statement, American Airlines said: "At this time American Airlines is unable to confirm whether or not the image referred to was taken by a member of flight crew but will investigate the situation internally to determine if any of the airline's strict policies were not correctly applied.

"American Airlines’ primary concern is for the safety and comfort of its passengers and crews and consequently passengers are advised to book two seats if they are concerned that they will require them. If a flight is not full, however, passengers' needs would be accommodated without charge wherever possible."